1. Field of the Invention
The invention relates generally to the design of breakaway posts for guardrail terminals and the like. More particularly, the invention relates to steel breakaway post design. In particular aspects, the invention relates to devices and methods for releasing a guardrail tension cable during a collision.
2. Description of the Related Art
A guardrail installation should be installed along a roadside or median such that its upstream end, or the end of the guardrail installation facing the flow of traffic, does not in itself form a hazard. As used herein, the term xe2x80x9cupstreamxe2x80x9d refers to the direction from which an impacting vehicle would be expected to approach. The term xe2x80x9cdownstreamxe2x80x9d refers to the opposite direction, i.e., the direction toward which an impacting vehicle would be expected to travel.
Many of the current generation of NCHRP Report 350-compliant guardrail terminals use wooden breakaway posts within the terminal section. The two most upstream end posts are typically wooden breakaway cable terminal (BCT) posts that are inserted into steel foundation tubes and sometimes joined with a ground strut to provide additional anchorage. The furthest upstream wooden breakaway post serves the dual function of supporting the rail element and securing one end of a tension cable proximate the ground line. Examples of this type of cable anchorage arrangement maybe seen in U.S. Pat. No. 5,547,309 issued to Mak et al.; U.S. Pat. No. 5,078,366 issued to Sicking et al.; U.S. Pat. No. 5,407,298 issued to Sicking et al.; and U.S. Pat. No. 5,391,016 issued to Ivey et al. Each of these arrangements disposes one end of the tension cable through a drilled hole in the lower portion of the wooden post. Wood has been used for breakaway posts because it is readily available and inexpensive. However, wood also has many drawbacks. For example, there are wide variations in the quality of the wood used, and thus the associated force necessary to break the post away varies. Also, the strength of a wooden post is affected by many factors including post size, ring density, location and size of knots and cracks, species and moisture content.
The ability of lead posts to break away is important to minimize the potential for vehicle ramping and excessive decelerations to vehicles during end-on impacts. When wooden posts are used, the ability to break the post away is enhanced by drilling holes through the post proximate to ground level, thereby reducing the force required to break the post away during an impact. This practice exposes the untreated interior of the post making it more prone to deterioration from environmental factors such as moisture, heat and freeze and thaw cycles. Additionally, wooden posts disposed in foundation tubes can sometimes be difficult to remove from the ground after impact due to swelling of the wood. Broken wooden posts are considered to be an environmental hazard due to the presence of chemical preservatives used in the wood to control decay. Proper disposal of such accident debris is becoming a significant problem.
In an effort to address some of the shortcomings of wooden breakaway posts, a number of steel breakaway post designs have been developed. Such post designs are described, for example, in U.S. Pat. Nos. 5,988,598 and 6,254,063. In general, however, these posts have not received wide spread acceptance due to maintenance problems and a high initial cost. There is, therefore, a need to develop steel breakaway posts that can be used as an alternative to wooden breakaway posts in guardrail terminals and other applications.
Additionally, wire rope safety rail systems have recently been developed that require separate anchorages for multiple horizontally-disposed tension cables. Thus, there is an even greater need for a cost-effective steel breakaway post design that provides an effective means of anchoring a tension cable and selectively releasing it.
Slip splice connections have been used for support posts for traffic signs and light poles. To the inventors"" knowledge, however, they have not heretofore been used successfully for support of portions of guardrail installations. Additionally, prior art slip splice connections have relied upon posts having horizontal upper and lower slip, splice plates that are held together by a plurality of bolts. During an impact to the upper portion of the post, the slip splice is activated to disconnect the upper portion of the post from the lower portion by causing relative horizontal movement of the upper post portion with respect to the lower post portion. Examples of such slip splice connections are described, for example, in U.S. Pat. No. 5,481,835 issued to Bloom; U.S. Pat. No. 4,923,319 issued to Dent; and U.S. Pat. No. 5,855,443 issued to Faller et al. These slip splice connections are not suitable breakaway arrangements for use in guardrail installations. With a guardrail support post, as opposed to, say, a light pole or traffic sign support, the center of mass and rotation is located much lower. Additionally, the overall masses of the traffic sign and light pole are significantly greater than that of the guardrail support post. As a result, the slip splice connection point on a guardrail support post is subjected to greater moment forces than translational forces and inertial forces. The lower end of the guardrail support post will, therefore, not xe2x80x9ckick outxe2x80x9d to the degree that the lower end of a sign support or light pole would. Thus, disconnection of the guardrail support post is much more uncertain.
An effective steel breakaway post would provide a number of advantages over wooden posts. Steel is a homogenous material with known characteristics that can be controlled. Thus, the strength and forces required to break the post away can be controlled and will provide better and more consistent safety performance for roadside devices. Steel is also more resistant to deterioration than wood in field applications and can be recycled to eliminate disposal problems. A well-designed breakaway steel post can be more economically installed by drivers as opposed to the drilling and backfilling required to install wooden posts. Further, breakaway steel posts would provide a more uniform appearance when terminals are attached to steel post guardrail systems.
The present invention addresses problems of the prior art.
The invention provides a cable release anchor having an improved breakaway post design with a cable release feature. In a preferred embodiment, the cable release anchor is fashioned of steel and used as the furthest upstream post in a guardrail terminal. The cable release anchor includes upper and lower anchor portions that are readily separable from one another during an impact. A bearing plate is retained within the end of each of the upper and lower anchor portions, and each bearing plate has a bearing surface that is oriented at an acute angle with the vertical when installed in the ground. Each of these bearing plates contains a U-shaped cutout. When the upper and lower anchor portions are joined, the cutouts form an opening through which an end of the tension cable is disposed. An impact to the upper anchor portion readily releases the upper post section from the lower post section and frees the cable.
The design of the present invention is advantageous in that it permits the upper anchor portion to present a different and smaller cross-section than that of the lower anchor portion. Thus, stronger anchorage for the release anchor is provided while still allowing the device to be driven into the ground using standard driving equipment.
End-on impacts to the guardrail installation will strike the upper anchor portion upon its weak axis. Disconnection of the splice connection of the upper and lower post sections then occurs without the binding that might tend to occur with a slip splice connection. When the length of the guardrail terminal is impacted, the angled bearing plates incorporated into the post provide positive anchorage for the guardrail for redirecting the impacting vehicle.